Curable epoxy compositions are often provided as a two-part formulation in which the epoxy resin is separated from the curing agent until immediately prior to the formation of a cured composition. Once mixed, the curing agent and the epoxy resin react quickly at room or elevated temperatures. Such curable epoxy compositions tend to have good storage stability (such as one year or more) but need to be used soon after the part containing the epoxy resin is mixed with the part containing the curing agent. Further, the two parts must be carefully metered together for mixing so the amount of the epoxy resin and curing agent are appropriate.
The two-part formulations can include various curing catalysts (e.g., accelerants) that can replace the curing agent or that can be used in combination with the curing agent. These compounds typically do not get consumed in the curing reaction but lower the activation barrier to polymerization of the epoxy resin with itself or with the curing agent. Typical curing catalysts include, for example, various imidazole compounds, urea compounds, tertiary amines, and Lewis acids.
Some one-part compositions are known in which a latent curing agent and/or latent curing catalyst is used. Although no mixing is required, the shelf-life of one-part systems typically is significantly reduced compared to two-part formulations. The latent curing agent and/or latent curing catalyst is often a blocked imidazole compound. Shelf-lives of 6 months or more can be achieved through the use of latent curing agents that are thermally activated to form the cured composition. The cure temperature is often limited by the melting point of the curing agent, which typically exceeds about150° C. or about 170° C. for conventional latent curing agents.